Sea water desalination is emerging as an important source of portable water in satisfying the increasing demand for water globally. Interests in ocean thermal energy conversion (OTEC) as an alternative energy source have been spurred on not only by the skyrocketing energy demand and prices but also by the need to find new renewable sources of clean energy in light of global warming concerns.
FIG. 1 shows a diagram of a heat engine 100. As shown, heat (QH) from a high temperature (HT) reservoir 110 flows through the engine to a low temperature (LT) reservoir 120. Heat is transmitted in the form of a fluid or vapor through a generator 140 to produce work (W). The theoretical maximum efficiency (η) of a heat engine is:
  η  =      1    -                  T        C                    T        H            where TC is the absolute temperature of the LT reservoir and TH is the absolute temperature of the HT reservoir. From the efficiency equation, it can be seen that the greater the differential between the temperatures of the reservoirs, the greater the efficiency of the heat engine.
For an OTEC system, it operates on the temperature differential between the ocean's surface and its lower depths. Generally, seawater at the ocean surface is about 28° C. (301 K) while the seawater at the subsurface, typically about 1000 meters (m) or beyond, rarely exceeds 5° C. (278 K). Given these facts, the maximum theoretical efficiency of an OTEC system is about 7.5%. With such a low efficiency, the various components of the system must be designed to operate at extremely high efficiencies to produce a net power gain. However, conventional OTEC systems require seawater to be pumped from the subsurface, typically from 1000 m or greater, to the surface. Since cold water is denser than warm water, pumping of seawater from such depths to the surface requires significant energy, which negatively impacts the already low efficiency or yield of the system. Additionally, the volume of deep seawater needed puts extreme stress on the pipes, creating reliability issues because of the large difference between the latent heat of vaporization and the specific heat and also the small difference in temperature between the vapor and the cooling water.